1. Field of the Invention
The present invention relates to an optical communications module for use in an optical transfer system for bidirectionally transferring a light signal through an optical fiber, the optical communications module having a light signal transmission capability or a light signal transmission/reaction capability; and a method for mounting an optical communications module. More particularly, the present invention relates to an optical communications module capable of utilizing a portion of light which is emitted from a front facet of a semiconductor laser device and is coupled to an optical fiber as monitor light so as to generate an optical output control signal; and a method for mounting such an optical communications module.
2. Description of the Related Art
In recent years, there have been proposed various optical subscriber network systems for transferring multi-channel video information and/or data from a central station to households, and ways for putting such systems to practical use have been studied. Such a system requires each household to install a plurality of optical reception modules as well as an optical transmission module having a light emission capability or a light emission/reception capability. A plurality of optical reception modules are necessary for simultaneously receiving different types of light signals which are transmitted across a wavelength division multiplexing optical network system. Therefore, there is always desired a cheaper and higher-performance optical reception module. On the other hand, an optical transmission module is necessary for transmitting requests and/or data from each household to the central station. Therefore, there is always desired a cheaper, smaller, and yet higher-performance optical transmission module.
Furthermore, an optical communications module for use in such systems, having a light signal transmission capability or a light signal transmission/reception capability, needs to be designed so as to be installable in any desired place. Specifically, an optical communications module to required to have excellent temperature characteristics for greater flexibility in the selection of installation locations. In particular, the tracking error characteristics with respect to optical output are very important to the stable transmission of signals.
An example of an optical transmission/reception apparatus for the aforementioned purposes is disclosed in the proceedings of the Institute of Electronics, Information and Communication Engineers Spring Conference in Japan, 1997, SC-3xe2x80x943. FIG. 13 is a plan view showing an optical transmission/reception apparatus 800.
First, the structure of the optical transmission/reception apparatus 800 is described. At a common port 88 and an output port 89 on a PLC (Planar Lightwave Circuit) substrate 81, external transfer paths (optical fibers) 90a and 90b are coupled to PLC waveguides 91a and 91b, respectively. The external transfer paths 90a and 90b are disposed in a fiber connection block 87. At a WDM (wavelength division multiplexing) filter 85, the PLC waveguides 91a and 91b are combined into a PLC waveguide 91c, which is again split into PLC waveguides 91d and 91e at a Y-juncture 86. The PLC waveguides 91d and 91e are coupled to, respectively, a photodiode element 83 for a 1.3 xcexcm wavelength band and a semiconductor laser device 52 for a 1.3 xcexcm wavelength band. According to this technique, the semiconductor laser device 82 is equivalent to an optical communications module having a transmission capability, and the photodiode device 83 is equivalent to an optical reception module. Behind the semiconductor laser device 82, a waveguide-type photodiode 84 for optical output monitoring purposes is provided on the PLC substrate 81. The conventional optical transmission/reception apparatus 800 is thus constructed.
The optical transmission/reception apparatus 800 receives light in the following manner: First, multiplexed light including a 1.3 xcexcm wavelength component and a 1.55 xcexcm wavelength component is input from the external transfer path 90a to the common port 88. Among the two light components, the light component of the 1.55 xcexcm wavelength band is reflected by the WDM filter 85 so as to be output to the external transfer path 90b via the output port 89. The other light component of the 1.3 xcexcm wavelength band is transmitted through the WDM filter. 85 and split at the Y-juncture 86 so as to be received by the photodiode device 83 for the 1.3 xcexcm wavelength band.
The optical transmission/reception apparatus 800 transmits light in the following manner: The light which is emitted from the front facet of the semiconductor laser device 82 for the 1.3 xcexcm wavelength band (which is a transmission light source) is optically coupled, without using any lens system, into the PLC waveguide 91e and propagated therethrough. This light undergoes an attenuation at the Y-juncture 86 in accordance with its branching ratio, and thereafter is propagated through the PLC waveguide 91a. Next, this light is transmitted through the WDM filter 85 and output to the external transfer path 90a via the common port 88.
Herein, the xe2x80x9cfront facetxe2x80x9d of the semiconductor laser device 82 refers to a face which is optically coupled to the waveguide 91e. A xe2x80x9crear facetxe2x80x9d refers to the opposite facet of the semiconductor laser device 82.
The above-described configuration of the conventional optical transmission/reception apparatus 800 is suitable for surface mounting, utilizing passive alignment, except for the junction portions between the external transfer paths (optic fibers) 90a and 90b and the common port 88 and the output port 89 on the PLC substrate 81.
In accordance with the conventional optical transmission/reception apparatus 800 shown in FIG. 13, a signal which is utilized for optical output control is obtained by the use of the optical output-monitoring waveguide-type photodiode 84. Specifically, the light which is emitted from the rear facet of the semiconductor laser device 82 is received by the optical output-monitoring waveguide-type photodiode 84, and a photocurrent which is generated responsive to the received light is utilized as a signal for optical output control.
In accordance with the conventional optical transmission/reception apparatus 800, it may be difficult to equalize the temperature characteristics (front facet temperature characteristics) of the coupling efficiency between the semiconductor laser device 82 and the PLC waveguide 91e with the temperature characteristics (rear facet temperature characteristics) of the light-current conversion efficiency of the optical output-monitoring waveguide-type photodiode 84 receiving the light which is emitted from the rear facet of the semiconductor laser device 82. This may lead to deterioration in the tracking error characteristics.
Examples of semiconductor laser devices for use in the above-described class of optical communications modules include spot size conversion laser devices and narrow divergence angle laser devices. In general, the radiation angle of laser light which is provided by a semiconductor laser device is known to have some dependency on the temperature of the semiconductor laser device. Furthermore, the radiation angle-temperature characteristics of the laser light which is emitted from the front facet of a semiconductor laser device (hereinafter referred to as the xe2x80x9cradiation angle-temperature characteristics on the front facetxe2x80x9d) may have discrepancies with the radiation angle-temperature characteristics of the laser light which is emitted from the rear facet of the semiconductor laser device (hereinafter referred to as the xe2x80x9cradiation angle-temperature characteristics on the rear facetxe2x80x9d). In particular, semiconductor laser devices such as narrow divergence angle laser devices, which provide an enlarged spot size by employing an active layer having a tapered configuration, are likely to have some discrepancies between the respective radiation angle-temperature characteristics on the front facet and the rear facet in a relatively wide range of temperatures, e.g., about xe2x88x9240xc2x0 C. to about 85xc2x0 C. In the case of conventional optical transmission/reception apparatus which utilizes the light emitted from the front facet as a transmission signal and which utilizes the light emitted from the rear facet as an optical output control signal (serving as monitoring light) for the semiconductor laser device, any substantial discrepancies between the respective radiation angle-temperature characteristics on the front facet and the rear facet will make it difficult to accurately monitor the optical output of the light emitted from the front facet of the device based on the light emitted from the rear facet of the device. Thus, discrepancies between the respective radiation angle-temperature characteristics on the front facet and the rear facet can be another cause for deterioration in the tracking error characteristics of the conventional optical transmission/reception apparatus 800.
In the case where multiple reflection occurs between the front facet of the semiconductor laser device and the facet of a waveguide or optical fiber, the semiconductor laser device may undergo a so-called mode hopping, so that the coupling efficiency with the waveguide or optical fiber may vary greatly, resulting in severe deterioration in the tracking error characteristics.
In order to prevent mode hopping from occurring, the conventional optical transmission/reception apparatus 800 is typically required to incorporate an antireflective film which is provided on the facet of the optical output-monitoring waveguide-type photodiode 84 adjacent to the semiconductor laser device 82, thereby preventing reflected light from returning to the semiconductor laser device 82.
In general, the mounting margin for the waveguide-type photodiode 84 with respect to the semiconductor laser device 82 is an the order of 5 xcexcm. This margin, which is extremely small relative to the mounting margin for photodiodes of a surface incidence type, is one cause for the relatively high mounting cost in the conventional structure.
Furthermore, as described above, the conventional optical transmission/reception apparatus 800 incorporates a PLC substrate 81 as an optical circuit. In the case where a PLC substrate (e.g., a PLC substrate of a silica type) is used, the chip size will be constrained by the minimum beding radius of the waveguide, as described in more detail below.
A given PLC waveguide has associated therewith a minimum beding radius above which the PLC waveguide does not incur any loss due to the difference in refractive index between the waveguide layer and the cladding layer. The minimum beding radius can be reduced by increasing the difference in refractive index between the waveguide layer and the cladding layer. For example, the minimum boding radius can be reduced to about 5 mm by increasing the aforementioned difference in refractive index to about 0.75%. However, the minimum boding radius cannot be further decreased from about 5 mm because the aforementioned difference in refractive index must not exceed 0.75%, which marks a practically maximum value in terms of internal loss within the waveguide and coupling loss with the optical fiber. It is generally understood that this constraint on the minimum beding radius is a cause for a long module size along the direction of light propagation, which hinders the downsizing of the module.
In the case of a bi-directional optical communications module 800 as shown in FIG. 13, which includes the fiber connection block 87 as well as the PLC portion, the size (the length along the optical axis direction) of the PLC portion alone measures at least about 15 mm. Hence, the size (the length along the optical axis direction) of the entire optical transmission/reception apparatus must at least be about 20 mm, including the fiber connection block 87 as well as the PLC portion.
According to the present invention, there is provided an optical communications module for use in an optical transfer system for bidirectionally transferring a light signal through an optical fiber, the optical communications module having a light signal transmission capability, wherein the optical communications module includes: a semiconductor laser; an optical fiber coupled to a front facet of the semiconductor laser; a light splitting element for splitting light which is emitted from the front facet of the semiconductor laser and propagated within the optical fiber: and an output-monitoring photodiode for receiving a portion of the light which has been split by the light splitting element as monitoring light, and generating a photocurrent based on the monitoring light, the photocurrent being used for controlling optical output of the semiconductor laser.
In an embodiment of the invention, the optical communications module further including a high reflectance film on a rear facet of the semiconductor laser.
In another embodiment of the invention, the semiconductor laser is a laser device selected from a group comprising narrow divergence angle laser devices and spot size conversion laser devices.
In still another embodiment of the invention, the semiconductor laser comprises a DFB laser device.
In still another embodiment of the invention, the semiconductor laser is a high power laser device for use as a pump light source.
In still another embodiment of the invention, the optical communications module further includes an optical fiber-embedding type optical circuit, the optical fiber-embedding type optical circuit having the optical fiber being embedded in an optical fiber-embedding substrate.
In still another embodiment of the invention, the light splitting element comprises a half mirror which is inserted in the optical fiber-embedding substrate at an angle with respect to a central axis of the optical fiber, the half mirror lying in an optical path within the optical fiber.
In still another embodiment of the invention, the optical communications module further including: a reception photodiode mounted on an upper face of the optical fiber-embedding substrate; and a metal total reflection film provided on a bottom face of the optical fiber-embedding substrate, wherein the output-monitoring photodiode is mounted on the upper face of the optical fiber-embedding substrate, and wherein an external light signal propagated through the optical fiber is reflected from the half mirror so as to be received by the reception photodiode, and the monitoring light is reflected by the half mirror and the metal total reflection film so as to be received by the output-monitoring photodiode, whereby the optical communications module has an optical signal transmission/reception capability.
In still another embodiment of the invention, the optical communications module further including a reception photodiode mounted on an upper face of the optical fiber-embedding substrate, wherein the output-monitoring photodiode is mounted on a bottom face of the optical fiber-embedding substrate, and wherein an external light signal propagated through the optical fiber is reflected from the half mirror so as to be received by the reception photodiode, and the monitoring light is reflected by the half mirror so as to be received by the output-monitoring photodiode, whereby the optical communications module has an optical signal transmission/reception capability.
In still another embodiment of the invention, the output-monitoring photodiode is mounted on an upper face of the optical fiber-embedding substrate, and wherein the monitoring light is reflected by the half mirror so as to be received by the output-monitoring photodiode, and wherein the half mirror has a reflectance in a range of about 5% to about 15% with respect to an emission wavelength of the semiconductor laser.
In still another embodiment of the invention, an interspace between the optical fiber-embedding substrate and a light-receiving face of the output-monitoring photodiode is substantially filled with a resin whose refractive index is matched with a refractive index of a material composing the fiber embedding substrate.
In still another embodiment of the invention, the optical communications module further includes an antireflective film with respect to an emission wavelength of the semiconductor laser in a light-incidence region of a light-incidence face of the output-monitoring photodiode.
In still another embodiment of the invention, the optical communications module further includes a metal light-shielding film on a surface of the output-monitoring photodiode in regions except for a light-receiving region and a light-incidence region for allowing incident light to pass through.
In still another embodiment of the invention, the optical communications module further includes a light-shielding resin substantially covering facets other than an upper face and a bottom face of the output-monitoring photodiode.
In still another embodiment of the invention, wherein the optical communications module is mounted in a package, and wherein the output-monitoring photodiode mounted on the bottom face of the optical fiber-embedding substrate is located within a concavity formed in a bottom face of the package.
Thus, the invention described herein makes possible the advantages of (1) providing a low-cost, compact, and high-performance optical communications module having improved tracking error characteristics; and (2) a method for such an optical communications module.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.